I didn't really understand Gaius' motivation for killing the Moonblooded people. Maybe I missed something but it seemed to me that Gaius just wanted to wipe them out for the sake of wiping them out? I thought it would've been cooler if they were evil in the past, forcing Dust to choose a middle path instead of wholeheartedly accepting and becoming Jin. Just my thoughts.

Yeah, I would have liked more of a backstory for General Gaius other than the 'oh i hate these people so ill wipe them out' sort of deal. It would have made excellent story development and would have made the world a bit more immersive in a sense.

Yeah, one of the few gripes I have about the game is how the ending motivation/plot doesn't quite mesh with the nicely interwoven plots/storytelling of the earlier chapters. Apparently this had something to do with deadlines and being unable to finish things as fully wanted. Which sucks.

General Gaius said something about being the "superior race" towards the end of game (80% sure).

However, there is no elaboration and as a result it felt more like an excuse than a genuine reason to hate an entire species. It would appear that he simply has no motivation at all. Despite this, Gaius still chooses to kill himself instead of giving in and stopping the genocide he (most likely) created.

Gaius may have had a grudge against the Moonblood at some point in time, but that is no longer important to him. Instead he simply wants to prove that he is right and that his actions are justified and would rather die than give in and admit that he is in the wrong. Even his (former) closest friend is unable to save him.

The story of Gaius is rather tragic. He is an antagonist that appears to be a good person overall, but has unfortunately allowed his weakness to destroy him.

Ok done. This is just one theory. I don't believe I saw anyone else say something like this so I thought I would give it a shot.